Somebody was on Sulawesi before 118,000 years ago

14 Jan 2016

Gerrit van den Bergh and colleagues reported in Nature this week that they have recovered artifacts from an early habitation of Sulawesi. Like Flores, Sulawesi was isolated from the mainland of Southeast Asia throughout the Pleistocene, so any humans who reached the island must have crossed Wallace’s Line by boat.

After the discovery of human remains at Liang Bua on Flores, and the much earlier archaeological evidence from the Soa Basin (“Earlier arrival of stone tools on Flores”), many speculated that hominins may also have reached Sulawesi at a similarly early date. The water crossing from Borneo to Sulawesi during the Pleistocene lowstand was substantially longer than the crossing from Bali to Lombok, but the Makassar Strait is long and parallel-sided, making a successful crossing relatively likely. A wider range of terrestrial mammals made the crossing to Sulawesi during the Pleistocene when compared to Flores, including macaques and pigs. All things considered, Sulawesi seems more likely human territory than other parts of Wallacea.

The late Mike Morwood set out to test the hypothesis by investigating stone tool industries on the island, and this work has been continued by van den Bergh and colleagues. Their current paper revolves around four artifacts recovered from a deep trench near the village of Talepu.

From T4 [Trench 4] we recovered 41 stone artefacts from the topsoil and colluvium down to a depth of 120 cm. However, four in situ silicified limestone artefacts were in exposed older strata within the silt of sub-unit E2 (Fig. 2c), and provide the stratigraphically earliest evidence for human activity at Talepu. Two are unmodified flakes (2.2–2.4 m depth) (Fig. 3l–m) and two are angular scatter fragments (3.0–3.1 m depth) (Fig. 3j–k), probably created in percussion flaking. The latter are made of a distinctive mottled silicified limestone and appear to have been removed from the same core. The artefacts bear no evidence of water transport; indeed, unit E did not yield any clasts indicative of high-energy water flow.

These artifacts are not diagnostic of any particular industry, so there is no possibility of connecting them to any culture. Their value is their depth.

The four samples analysed from T2 have ages in stratigraphic order 103 ± 9 kyr at 3 m depth to 156 ± 19 kyr at 10 m depth (Fig. 2, Methods: Optical Dating, and Supplementary Table 4). These results suggest that the Talepu cultural sequence ends at ~100 kyr, or possibly earlier (see Methods). The sediments dated to 156 ± 19 kyr were deposited near the top of unit D, which overlies the sedimentary layer (unit E) from which the deepest artefacts were excavated (more than 3 m below). The oldest securely dated evidence for stone artefacts at Talepu is, therefore, 118 kyr to 194 kyr in age at the 95% confidence interval (2σ), although human occupation of the site clearly occurred earlier given the recovery of artefacts from greater stratigraphic depths (Fig. 2c).

The deepest artifacts are tiny, a bit more than a centimeter in their longest dimension, and I wonder whether vertical movement of these flakes within the sediments may have occurred. Setting this possibility aside, the authors’ argument for a minimum age of 118,000 is otherwise very conservative—it marks the youngest end of the 95% confidence interval on a sediment layer that substantially overlies the unit containing the four deep artifacts. These four deep artifacts within unit E2 may be much older.

The paper ends with a discussion of who made the tools. The authors consider that early modern humans may have been in southern China before 100,000 years ago, so it seems barely possible that a very early modern human migration may have reached Sulawesi by 118,000 years ago. They discuss the hypothesis that an earlier population may have been present, by analogy with Flores.

In my view, it is premature to think that we understand much if anything about the first appearance of modern humans in Southeast Asia. There are too few pieces of evidence. None of the fragments yet found in China from earlier than 80,000 years ago are very persuasive of affinity with modern humans. They aren’t Neandertals, but in truth we have no idea what other populations of later archaic humans may have looked like, how much genetic diversity they may have had. Conceivably, a very early emergence of African or West Asian people—maybe even those evidenced at Qesem Cave—may have spread into India or Southeast Asia, bringing signs of affinities with a more modern-like population. We have no reason to assume that other populations, such as the Denisovans, would not be mistaken for modern humans, certainly based on the fragments that have so far been unearthed.

We know now that the Neandertals were arrayed across the western half of Eurasia, in a set of populations that collectively were highly endogamous and subject to strong genetic drift. Across a broad geographic range, such high inbreeding could only be accomplished by high mobility on a millennial timescale, a population structure in which no long-isolated population persisted for long. This was a dynamic and adaptable population, probably relying upon ecotones at the edges of the wide Eurasian steppes, seemingly more successful in hills, mountains and forest edges than in open country.

Southeast Asia, including Indonesia, is a very different biogeographic story from Europe and Central Asia. At low sealevel, Sundaland together with Indochina formed a continental area larger in habitable area than Europe. Across this region the most uniform habitat is the coastline, which sometimes, cut through the interior creating dispersal corridors, as it does today. Inland forested areas in historic times have remained islands of high faunal and cultural endemism. These resource-rich environments persisted throughout the spread of rice agriculture, and are yielding only now to development, including industrial-scale palm oil plantations. It seems unlikely that the entire region was home to a single Neandertal-like population that maintained high inbreeding across its range by high mobility and dispersal. Instead, the vast land mass of Southeast Asia was probably home to many human populations of high regional endemism. The biogeographic islands within Southeast Asia may have retained some Homo erectus-like populations, but almost certainly the dispersal corridors across the region would have been dominated by events coming from the large population of the Indian subcontinent to the west.

I’m very enthusiastic about Sulawesi. It may be a beautiful test of the biogeography of early Homo across its southern range. If archaic humans were effectively using coastal habitat as a dispersal corridor, we may expect that they repeatedly reached Sulawesi—by 120,000 years ago, they may even have been in continuous contact.

Or if Southeast Asia was full of human populations with high endemism, some founded by Homo erectus-like populations, then Sulawesi may have been home to such a population. Unlike Flores, the resource base on Sulawesi was richer and island’s size would have enabled a relatively large human population, possibly large enough to avoid the mutational meltdown possibilities of the smaller island population.

Artifacts from a single deep trench do not establish either possibility, and so far the artifactual record across Southeast Asia as a whole gives hardly any hint of the cultural diversity that must have existed in the Middle Pleistocene. It is a large question mark. But the fact that artifacts turned up in a trench dug through sediments of the right age is promising. If the habitation of the island was intensive enough to salt the traces of human activity this densely into the earth, there is a good chance that further survey will turn up more interesting things.

John Hawks is the Vilas-Borghesi Distinguished Achievement Professor of Anthropology at the University of Wisconsin—Madison. I work on the fossil and genetic record of human evolution (About me).

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